It has long been known in the art that cellulose fibers used in the manufacture of paper and paperboard have a natural affinity for water even though they are water insoluble. It is this characteristic of cellulose fibers which makes possible the paper and paperboard manufacturing processes currently used. This water affinity has its advantages and disadvantages.
In forming paper or paperboard, the cellulose fibers are dispersed in a dilute aqueous slurry which is wet laid as a mat or web onto the screen of a conventional Fourdrinier-type machine. After the web has been dewatered, it is dried to a predetermined moisture level upstream of the size press, and after application of sizing or other treatment at the size press, the web is again dried to its finished condition. The drying process is known to affect the physical characteristics of the finished product, including its planarity, or flatness.
Even though the finished paper is dried, it always contains a certain amount of moisture. The amount of moisture will vary, depending upon the relative humidity to which the finished paper is subjected. Thus, except for a relatively narrow range of relative humidity conditions when the paper is in equilibrium with moisture in its ambient atmosphere, the paper is either taking on or giving off moisture. In other words, paper is hygroscopic.
Different papers have different degrees of hygroscopicity. In general, heavily refined fibers will absorb more moisture under the same relative humidity conditions than will less heavily refined fibers. In Volume III of PAPER MAKING AND PAPER BOARD MAKING, published by McGraw-Hill in 1970, the theory of drying is discussed, and the relationship between moisture content of various papers and ambient relative humidity is graphically illustrated. For instance, newsprint paper contains approximately 5% moisture at 50% relative humidity at 70.degree. F. Under the same conditions of humidity and temperature, kraft paper contains almost 7% moisture.
As noted in the aforementioned publication, water may be present in paper in any one or more of several forms. For example, the water may be chemically or mechanically bound to the fibers as hygroscopic water. It may be contained within the fibers, located on the surfaces of the fibers, or entrapped in the voids of the fiber network. The water may be chemically bound by having entered into a hydration bond with the molecules of the cellulose fibers or with other substances in the web, such as water soluble sizing agents in the surfaces of these molecules.
Because paper and paperboard are hygroscopic, they are affected dimensionally by changes in moisture content, particularly during drying. This is because as moisture is evaporated from the web, the web tends to shrink. However, because the shrinkage does not occur evenly, the web may develop cockles, wrinkles and a general bagginess.
The addition of sizing agents to the web often exacerbates the sensitivity of the paper to moisture changes. This is because sizing agents often exhibit greater moisture sensitivity, i.e. a greater tendency to expand or contract in a response to changes in moisture, than the cellulosic fibers themselves. This phenomenon is particularly pronounced when the web is treated with starches, polyvinyl alcohols, carboxymethyl cellulose, methyl celluloses, and carboxylated celluloses such as kelgin, and the like. As a result, the thus-treated sheets often exhibit poorer planarity, or flatness, than untreated sheets.
Because the aforementioned planarity problems are known in the art, it is customary to dry the web during manufacture to a moisture level which is intended to place it in equilibrium with the average relative humidity to which the finished paper is likely to be subjected, generally accepted to be 50%. This is because if the web is overdried, it will acquire excessive moisture and hence expand excessively when exposed to average humidity conditions. When in roll form, moisture pick up causes expansion or rope marks to form in the outer layers of the rolls, and when the paper is in the form of a sheet, it tends to form undesirable cockles, wrinkles, and to exhibit a general bagginess. Hence, in order to compensate for these undesirable tendencies, paper is normally finish dried to a moisture content in a range of about 5% to about 6%, by weight, based on the weight of the finished paper.
It is known that there are occasions during manufacture when the paper web will have an uneven moisture profile in the cross-machine direction. These wet streaks, or other areas of high moisture content may exist even when the paper is finish dried to the 5-6% moisture range noted above. Some paper makers attempt to compensate for this uneven moisture profile by further reducing the finished moisture level, generally down to about 4%, to bring the variations in the moisture profile into an acceptable range. However, drying paper to this lower level is undesirable because it increases the brittleness of the paper. Moreover, finished moisture levels below about 5% are generally regarded as being undesirable because of the recognized proclivity of overdried papers to cockle, wrinkle and develop bagginess when the finished paper is subjected to relative humidity within anticipated ranges.
After manufacture, paper is often subjected to further treatment to impart various desirable properties, including moisture resistance, grease resistance, or heat sealing qualities. When these treatments are applied as a water-based coating on one side of the web, the paper will wrinkle or cockle more severely and develop more curl if the paper is too dry. In fact, any time paper is subjected to a moisture-containing treatment, either by way of coating or sizing, it is always difficult to keep the flatness of the final product within acceptable limits. For this reason, water-based coatings or treatments are often not used.
In some industrial applications for paper, the web may be subjected to heat in an oven, frequently for extended periods of time at elevated temperatures. Under these conditions, the paper will shrink and develop cockles. Sometimes, the paper will curl if the heat is applied unevenly. The paper also tends to become brittle. Furthermore, unless a controlled amount of moisture is again applied to the paper after leaving the oven, there is a tendency for the paper uncontrollably to reabsorb moisture and thereby lose the desired flatness.
Paper is sometimes laminated with plastic film or aluminum foil. However, if the paper is overdried prior to lamination, serious curl problems can develop. Often, reconditioning of the overdried paper is necessary before laminating, and even when these precautions are taken, inherent imbalance in the dimensional stability of the two components of the laminate tends to induce the laminate to curl in response to changes in relative humidity.